How Will Humans Fly to the Stars?

byPaul GilsteronApril 17, 2012

by Andreas Hein

The immense problems of time, distance and life support invariably mean that when we talk about an interstellar mission, we talk about robotics. But the imaginative team at Icarus Interstellar, which is now setting up projects on everything from beamed lightsails (Project Forward) to pulse propulsion engines (Project Helios), has pushed into the biggest what-if of all, the question of manned missions. And as project leader Andreas Hein reminds us in the following article, a variety of approaches have been suggested for this over the years from which a new concept study can grow.

Andreas Hein received his master’s degree in aerospace engineering at the Technische Universität München, and is doing his PhD at the same university in the area of space systems engineering at the Institute of Astronautics. He has participated in several mission studies: a lunar gravity measurement mission by EADS and a cubesat mission analysis. During his internship at ESA-ESTEC, he participated in the joint ESA/industry lunar architecture study of the human spaceflight division, applying different systems engineering methodologies such as stakeholder analysis. Andreas is currently supervising a course on concurrent engineering of space systems at the Institute of Astronautics whose objective is to design an Earth observation mission. His particular interest is in the early phases of systems design, which he lists as requirements engineering, functional analysis, concept design/trade offs.

News of recently discovered exoplanets reaches us almost weekly and it seems only a matter of time until an “Earth 2.0” is found out there. If this happens, is there any possibility for humans to travel to this planet with today’s or foreseeable technologies? This is the main question Project Hyperion deals with.

Project Hyperion is a research project whose main purpose is to assess the feasibility of crewed interstellar flight with current or near-future technologies.

I will first give a brief overview of existing concepts for crewed interstellar flight and then dwell on challenges apart from crossing the distance between the stars.

Image: Andreas Hein (left) and Alan Bond, designer of the Bond/Martin World Ship concept, at the headquarters of the British Interplanetary Society. Credit: A. Hein.

Ideas for the Long Haul

The primary purpose of crewed interstellar flight is the long-term colonization of an exosolar system. What are the concepts that exist today to achieve this goal? The most well-known concept was introduced by Robert Forward in 1984 [1], who discusses the feasibility of a crewed trip to the nearest stars by a laser-pushed sail. Another concept is the fusion-propelled “colony ship” by Gregory Matloff, published in 1976 [2] and the “world ship” concept of Alan Bond and Anthony Martin [3]. A recent analysis for an antimatter rocket was conducted by Robert Frisbee in [4]. Most of the existing “near-term” concepts can be put into four categories, based on the mode of crew transportation:

Crew lives on the spacecraft: world ships, colony ships [2-5]

Crew is in suspended animation or hibernates [6]

Crew is transported as embryos or single cells [7]

Crew is transported digitally [8]

We still know too little about each of these concepts. Nevertheless, some educated guesses can be made: From a) to d) an increasing level of technological sophistication is required in order to achieve the mission. On the other hand, the resources needed for transporting the crew decrease at the same time, which enables faster travel. Think of a lorry being slower than a Ferrari although they have the same horsepower. A world or colony ship is a massive spacecraft, several tens of miles-long. The crew lives and dies over generations on these small worlds, crossing the comic ocean between the stars taking centuries.

Image: A worldship dwarfs the imagination, offering a habitat for humans over the course of generations on a long, slow voyage to the stars. Credit: Adrian Mann.

A suspended animation / hibernation ship is still large but does not require a habitat as large as a world ship: most of time the crew “sleeps” during the trip like in the introductory sequence of the movie “Avatar”. However, some kind of habitat is probably still required for periodical “awakenings”. An embryo or single cell ship will likely have a payload the size of a house as no habitat is required and a digital crew spacecraft payload might have the size of a small car or even smaller.

Each concept is challenging in its own way: It is in principle possible to build a world ship or colony ship today, given the resources. However, how to design a habitat that enables human survival and comfort for centuries? This has analogies to people living on an island cut-off from the rest of the world. How will culture and technology develop under such conditions? Will people live like in the Stone-Age when they arrive at the target star system? How many people should be sent to maintain at least a certain level of cultural and technological diversity? How is the spacecraft maintained over centuries?

The feasibility of using a spacecraft with a crew in suspended animation or hibernation depends on the applicability of these concepts to humans. Whether or not it is possible to sustain a crew in this state for decades or even centuries is still an open question.

An embryo or single cell ship needs an approach to raise and educate them at the target system. Methods like android parenting have been mentioned in the literature. Furthermore, sophisticated automatic manufacturing is required to construct a shelter and to provide nourishment on the target planet. These are all technologies that are not available today. Whether or not a crew can be “resurrected” on the basis of digital data in the target system is an open question. If this becomes possible one day, this technology will not only make this special form of interstellar travel possible but will have a profound impact on our life on Earth.

Besides these technological challenges for each concept, different ethical questions have to be addressed as we are dealing with decisions that have far reaching consequences for the crew sent out to the stars and the ones that are yet unborn.

Building the Star Colony

We have now briefly covered the potential approaches for crossing the gap between the stars. But what happens once the crew arrives at the target star system?

Most of the scientific literature focuses on the trip between the stars. However, the establishment of a colony within the target star system is a neglected, but vital part for planning such a mission [9]. We often assume that a habitable exoplanet is very similar to Earth and human life is possible on its surface. But there are myriads of potential obstacles to colonizing such a planet: An ecosystem that does not provide edible food and water, diseases to which the human immune system can not adopt adequately, toxic substances etc.

It is difficult to say whether it is possible to anticipate these difficulties in advance as many issues will remain undiscovered, even after an exploration with precursor probes. Alternative approaches like terraforming and building artificial colonies in space were also considered [2, 9]. Terraforming aims at changing a planet in a way that human life is possible on its surface for extended periods of time. Artificial colonies might be the last fall-back-option, in case human life is not sustainable on a planetary surface. The concept of large space colonies has been proposed by the physicist Gerard O’Neill in the 70s as a mode of human existence in the future [10]. This might ultimately make colonization of another star system independent of the discovery of a habitable exoplanet. However, the immense difficulties associated with both approaches are difficult to anticipate today.

Image: The ISV Venture Star from James Cameron’s film Avatar. Credit: Ben Procter.

Most of the publications mentioned in this article have limited detail and are restricted to rough outlines and estimates without detailed engineering assessments. Think of the difference between a painter’s pencil sketch to prepare for a painting and the meticulous work that has to be put in to flesh out all the details on the canvas. Currently, only rough sketches exist.

With Project Hyperion we want to get from the sketch to the real painting! It is our conviction that today most of the required feasibility analysis for a crewed interstellar spacecraft can be done with current knowledge. Some readers will certainly think: “Ok, this is a nice exercise but won’t it take several centuries to realize such a mission? So why think about this today?” I agree that it is still a long time to go to see the first humans heading to the stars. However, we humans exist on Earth as a species for about 200,000 years. If one thousand years are one meter, this is a distance of about 200 meters, more than twice the length of a football field. If it takes about 300 years to send out the first humans to the stars, on this scale, this is about 30 cm. How insignificant in comparison to the distance in time we already traveled as a species and what a magnificent chance to take the first steps towards this goal today.

I disagree that it will be hard to seed a world with life. It is a matter of finding an autotroph that will thrive in some niche on that world, preferably a large one like the open ocean. There are plenty of hardy autotrophs capable of surviving under a variety of conditions here on Earth. Selecting one or several that are suitable for a given habitable (in the widest sense) planet should be quite doable. More so if genetic engineering is applied. The idea that you need a whole ecosystem is wrong, plenty of organisms can be cultured in isolation. Ecosystems will develop, in time, as evolution takes hold.

The hard part is predicting or trying to control what happens after. Hosting life is not the same as being habitable for humans. It may take billions of years for atmospheric oxygen to stick around, as it did on Earth. All sorts of horrible, gooey things may happen in between. The climate may go south (or north) from the changes, in unanticipated ways. Thinking of it, it almost seems like a non-starter to me, now.

Eniac, those bacteria growing in isolation are usually using a biologically derived medium, such as agar, that may vital traces of coenzymes that they can’t synthesis themselves. Even with an autotroph growing in a pure chemical medium, I am struck by two more problems.

Firstly some microbes are many times better at utilising a limited resource in stringent mode that in their optimal growth mode. I recall, that even for such high usage material as nitrogen, some yeasts had been thought to be capable of nitrogen fixation, due to their incredible ability to continue growing when their colonies transferred to media without fixed sources.

Secondly, most bacterial colonies that are allowed to continue exponential growth by continually transferring the to new media, seem to undergo significant genetic deterioration from the wild strain. I have heard this several times, but have no peer reviewed source for it.

These would mean that proving that any autotroph is truly independent would be tricky.

Eniac: “Twenty independent space colonies which can serve as each other’s refuge will be less likely to go extinct than a single planet, because they cannot all be hit by catastrophe at the same time.”

No, I do not think so: they can all suffer from similar technical weaknesses and failures, and/or similar internal and external threats (though maybe not exactly the same occurrence, but different occurrences of such a threat).
On a planet, re-colonization from unaffected areas can nearly always take place and planet-wide extinction events are (despite recent impressions given by movies) extremely rare.
I agree with you that a complete wipe-out of all those space structures would be statistically highly unlikely, but the same is even more valid for planet-based colonies.
Anyway, the cost of maintenance, repair and replacement of such space colonies will be and remain extremely high.

Anyway, the cost of maintenance, repair and replacement of such space colonies will be and remain extremely high.

This seems like a reasonable assumption, but I have my doubts about it. I am fairly sure that the dawning age of autofabrication will make anything material really, really cheap, in just a few more decades of time. Cheap labor in developing countries will soon be replaced by autofabrication, and if you thought Chinese goods were cheap, you ain’t seen nothing yet….

Humanity really is coming to a crossroads, one that in a matter of decades, probably before this century is out, will do one of the following: Change everything for the better for all of us, change for the better for a new form of intelligence and maybe humanity if it can handle things (and if the new intelligence has a place for us), or we will miss the technological train and it will be a slow wait stuck at the station to degeneration and possibly extinction.

This is not just one of those Singularity things. We need to make some kind of change NOW (actually yesterday) because I am watching 7 billion people and growing every second still acting and living like Earth is this infinite plain just chock full of never ending land and resources. Not to mention that a powerful deity says this is all ours and we can do whatever we want to it and any “lower” creatures upon it.

And meanwhile the talk of expansion into space is still confined to the minority group of space advocates while the politicians (the ones with the money and influence) and the general public laugh at guys like Newt Gingrich who said we should put a manned base on the Moon (with Romney saying he would fire anyone who came to him with such a proposal – nice).

So with thinking and acting like that, I see little choice other than we start radically changing things to have a Star Trek style future (sans warp drive and mostly mean aliens who look and act a lot like humans) or we can eventually have the authorities tell us that Tuesday is Soylent Green Day.

Because anyone with any sense knows we cannot keep our current society going without consequences down the road, at least certainly for our children. And if you think 7 billion people can and will go back to the “simple” life of being one with nature without massive problems, oh boy.

And if we cannot survive on a “spaceship” eight thousand miles across, how well will we do in a tin can a few miles in diameter on a very long one-way journey to uncertainty?

…Living organisms being an obvious example of such well developed autofabrication. Without FTL transport, human engineers and their assistants have hundreds or thousands of years of development time while the first starship is in transit. Isn’t it a reasonable bet that “we” can do it better than Darwinian evolution and in much less time? Just consider the “directed evolution” of microelectronics over the last 50 years.

Eniac, ljk – I am glad the cat fight over my off-topic remark is over and we all agree. They might be here but probably they are not. I did notice, though, that “ljk” apparently assumed that if they were here then they had some interest in us. I would never make that assumption.

{BeginWhisper} But that could change when the pink and brown goo begins to leak badly from the inner blue planet. Then, the Matrioshka Elves may release the Trolls into the inner solar system and things will get sticky. What side will the Angels be on? {EndWhisper}. I am so sorry. Hard as I tried, I just could not restrain myself

“Eniac, ljk – I am glad the cat fight over my off-topic remark is over and we all agree. They might be here but probably they are not. I did notice, though, that “ljk” apparently assumed that if they were here then they had some interest in us. I would never make that assumption.”

LJK replies:

First off: Cat fight?

While I do assume that any ETI who went to all the trouble of sending a vessel to our Sol system might have done so because they became aware of our presence in the first place and at least wanted to check us out up close in order to update their Encyclopedia Galactica (I refuse to become trendy and call it Wikipedia Galactica), I accept there may be other reasons, too.

Maybe they are more like giant jellyfish and they are really here to check on the balloon creatures floating in the atmosphere of Jupiter. Or maybe they just stopped to refuel ala Clarke’s Rendezvous with Rama. Or maybe they are part of a Kardashev Type 3 civilization and are in need of new building materials. See, my thoughts are not so that I think humans are or would be the focus of the galaxy. If we were so cosmically popular, we would likely have the ultimate answer to the ultimate question by now.

Warren then says:

“{BeginWhisper} But that could change when the pink and brown goo begins to leak badly from the inner blue planet. Then, the Matrioshka Elves may release the Trolls into the inner solar system and things will get sticky. What side will the Angels be on? {EndWhisper}. I am so sorry. Hard as I tried, I just could not restrain myself.”

LJK replies:

If aliens are not trying to take over our blue globe or capture our women, the next thing they often seem to be doing in science fiction is chiding us for being such poor stewarts of the planet and mean warriors.

Assuming for the moment that an ALIEN species (and I mean ALIEN, not Star Trek type alien which is often hardly anything but) would automatically recognize such human traits as “bad”, why are we being the ones punished, especially since it is only quite recently on a cosmic scale that we became aware of our true place in the Cosmos?

These superior ETI who ply the stars in their sophisticated starships with their highly advance technology and presumed equally superior morals and ethics – how did they get that way? Do you mean to tell me they were able to evolve into a technological civilization without breaking open a single rock or harming even one blade of alien grass on their home world or any other?
And as a society, did they all live in peace and harmony for their entire existence without one single conflict or struggle for survival?

So why didn’t another ETI superior to them come along and tell them to knock it off or else? How come they get to go around being the Galactic Policemen? Is it because they see humanity as a competitive threat to galactic resources and territory down the road and this police action is the less extreme way of taking us out of the race?

Unless humanity starts getting actively and obviously aggressive towards other beings in the galaxy, I have trouble imagining that an ETI would be all that concerned about our behavior in such a huge place like the Milky Way.

By the way, I know the reason that aliens in science fiction wag their fingers and tentacles at us to remonstrate us for our bad behavior is really a commentary for us by us to stop polluting, fighting, etc., but this literary message has often become a seriously considered reason why ETI would take the time to contact us. I know, I have no absolute proof it would never happen, but I have given my reasons above why it seems unlikely. Besides, at least for us lowly humans, sometimes being aggressive is the only way to survive, such as going to war against a ruthless dictator. I would be more than a little wary about any “space angels” who came to “rescue” us.

“Isn’t it a reasonable bet that “we” can do it better than Darwinian evolution and in much less time? Just consider the “directed evolution” of microelectronics over the last 50 years. ”
When I said it would be difficult to seed a planet with life , the idea was that it would be difficult to do i RIGHT ,which to me means to get a measurable and detectable result as fast as possible . The good news is that this an area where experimentation can be done relativly cheap , all you need is a hothouse or a swimingpool that can be effectivly sterilized and isolated ,and the classical tools of microbiology. The bad news is that noboddy seems to be doing it , a total vacuum .
An endless list of good questions with no answers , heres a few exambels :
asuming an atmosphere with a high concentration of H2o and Co2 and no O2 , could any existing multicellular plants plants thrive without the support of a comlex microbial society delivering nutrients ?
And if no such plant exist , how difficult would it be to devellop one ?

When I said it would be difficult to seed a planet with life , the idea was that it would be difficult to do i RIGHT

We agree on this, then.

asuming an atmosphere with a high concentration of H2o and Co2 and no O2 , could any existing multicellular plants plants thrive without the support of a comlex microbial society delivering nutrients ?

I doubt it. I think pretty much all multicellular organisms are aerobic, and will not grow in high CO2 and the absence of oxygen, even plants. But microbes are much more chemically diverse, and it is my guess that for any reasonable planet in a HZ with liquid water and CO2, an organism can be found that can live in that water and photosynthesize. I would look among the cyanobacteria, for example.

The obvious solution is first to seed a planet with oxygenproducing bacteria and only later in a separate mission with higher plants , but many other possibilities exist , because plant seeds can be very longlived .
As the plants produce oxygen themselves by fotosyntesis , it would not be unreasonable to supose that prehistoric species had the ability to store this oxygen , or its chemical equivalent in some specialized organs ,but that most of this ability has been unnescsary for half a bilion years . Some waterplants stil devellop specialized tubers which help them survive a winterseason of anaerobic condtions .
Another possiblity would be a symbiotic relationship between a plant and cyannobacteria, perhans similar to the way many plants cooperates with nitrificationbacteria .

As the plants produce oxygen themselves by fotosyntesis , it would not be unreasonable to supose that prehistoric species had the ability to store this oxygen ,

Plants don’t really need oxygen, and many ways could be imagined in which they would deal with periods of darkness in other ways. There is no particular reason plants could not exist in pure CO2, except that it is not an environment they are adapted to.

Plants ARE the product of symbiosis with cyanobacteria, which gave rise to the chloroplast, a universal characteristic of eukaryotic plants.

Terraforming should never be attempted on places like Mars or even Venus. This for the simple reason that we cannot be sure if there is or isn’t life there already. This would be my first objection to anyone who suggest such a venture.

There is another more practical arguement against doing such a thing and that is the time frame involved. No entity (kingdoms, theocracies, governments and possibly sentient species) may last the length of time it would require for the process to come to fruition.

Lets try the tin can approach. Send enough of them out and some will take. A Darwinian approach of sorts…

“Terraforming should never be attempted on places like Mars or even Venus. This for the simple reason that we cannot be sure if there is or isn’t life there already. This would be my first objection to anyone who suggest such a venture.”

Suppose there are life forms on Venus and Mars: If they are no more advanced than, say, bacteria, and the human race needs to colonize Venus and Mars in order to survive – and terraforming them is the only way for our species to have a chance at living on them properly – should we leave these creatures alone?

With conditions so harsh on both worlds, any life there will be hanging on at best. They won’t be thriving and they certainly won’t be developing into anything more complex or interesting any time soon. This is the case with certain microorganisms found in the rocks in the interior of Antarctica. They are alive and they are hardy, but they are just surviving due to the climate there.

Now obviously such creatures, perhaps the first alien organisms humanity will come across, should be studied and preserved. Ironically it might be better for them to be kept on Earth or in some laboratory in a suitable space-based facility (I assume such things will exist by the time we are able to explore Venus and Mars in such a way that we can find life on those planets).

It would be more than conceivable that whatever life is on those worlds are the last survivors of earlier eras when things were a bit nicer in the climate department. So leaving them there even if humanity did not attempt to change the conditions on those planets might be a crueler fate than taking them off world.

The point is, humanity will expand into space, barring a disaster that renders us degenerate or extinct. Now Venus and Mars may never have had life at all, or if it did only fossils remain if that. But if there is life there, unless they are some strange variety that can strongly retaliate against any and all perceived threats to their existence (and that smacks more of science fiction than reality), they will likely be mowed over by the human colonizers. And unlike our past history with its many examples of one human group taking over the territory of another with often fatal consequences, this will not be some kind of deliberate act.

However, as I said above, if the choice comes down to human beings or microbes, even alien ones, who do think will win? And the minority of scientists (and they will always be a minority so long as human society remains essentially the same) who ask that the planets be left alone to keep their little remaining natives intact will be quickly overruled.

I do not say any of this out of a desire to see such a thing happen, I am only pointing out what likely will happen some day. When we start expanding into the galaxy and coming upon other worlds where the native organisms are decidely more complex and aware, that is when things may become very dicey.

Perhaps the solution is to leave certain worlds alone as nature preserves, assuming there are more than enough other planets to settle. Maybe that is how Earth is being treated right now, aka the Zoo Hypothesis.

Found this quote while reading about Walter M. Miller’s SF novel A Canticle for Leibowitz on Wikipedia, about humanity after a nuclear war through the centuries until civilization rises up again – along with nuclear weapons:

“In her analysis of Miller’s fiction, Rose Secrest connects this theme directly to one of Miller’s earlier short fiction works, quoting a passage from “The Ties that Bind”, published in the May 1954 edition of If magazine:

“All societies go through three phases…. First there is the struggle to integrate in a hostile environment. Then, after integration, comes an explosive expansion of the culture-conquest…. Then a withering of the mother culture, and the rebellious rise of young cultures.”[16]

The goal of the human spaceflight program should be to increase the survival prospects of the human race by colonizing space. Self-sustaining colonies in space, which could later plant still other colonies, would provide us with a life insurance policy against any catastrophes which might occur on
Earth.

Fossils of extinct species offer ample testimony that such catastrophes do occur. Our species is 200,000 years old; the Neanderthals went extinct after 300,000 years. Of our genus (Homo) and the entire Hominidae family, we are the only species left. Most species leave no descendant species.

Improving our survival prospects is something we should be willing to spend large sums of money on governments make large expenditures on defense for the survival of their citizens.

The Greeks put all their books in the great Alexandrian library. I’m sure they guarded it very well. But eventually it burnt down taking all the books with it. It’s fortunate that some copies of Sophocles’ plays were stored elsewhere, for these are the only ones that we have now (7 out of 120 plays).

We should be planting colonies off the Earth now as a life insurance policy against whatever unexpected catastrophes may await us on the Earth. Of course, we should still be doing everything possible to protect
our environment and safeguard our prospects on the Earth. But chaos theory tells us that we may well be unable to predict the specific cause of our demise as a species. By definition, whatever causes us to go extinct will be something the likes of which we have not experienced so far.

Good discussion.
Though – looking at the thoughts about travelling to Moon and planets 150 years ago ( or 15 cm as in the example above ) – the real travel will for sure be quite different to what is discussed today. We live in an world of extensive energy-needs. To bring up several times the energy of our whole civilization for a space-ship is far from speculation. It is not realistic at all. Maybe we will use things like worm-holes or shifting to parallel-worlds, But one day we will :-)

Abstract: The formation, composition and physical properties of lunar dust are incompletely characterised with regard to human health. While the physical and chemical determinants of dust toxicity for materials such as asbestos, quartz, volcanic ashes and urban particulate matter have been the focus of substantial research efforts, lunar dust properties, and therefore lunar dust toxicity may differ substantially.

In this contribution, past and ongoing work on dust toxicity is reviewed, and major knowledge gaps that prevent an accurate assessment of lunar dust toxicity are identified. Finally, a range of studies using ground-based, low-gravity, and in situ measurements is recommended to address the identified knowledge gaps.

Because none of the curated lunar samples exist in a pristine state that preserves the surface reactive chemical aspects thought to be present on the lunar surface, studies using this material carry with them considerable uncertainty in terms of fidelity. As a consequence, in situ data on lunar dust properties will be required to provide ground truth for ground-based studies quantifying the toxicity of dust exposure and the associated health risks during future manned lunar missions.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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